Wheel assemblies on Metro rail cars like the one at fault in last week’s Blue Line derailment had failed 31 times since 2017 — and renewed inspections last week identified almost two dozen similar defects, the chair of the National Transportation Safety Board said Monday.

The Washington Metropolitan Area Transit Authority pulled the 7000 series cars from service Sunday night, leaving the agency down more than half its fleet as service reductions and crowded trains were expected through the week.

Investigators said the train that derailed last Tuesday had previously derailed at least twice on the same day before leaving the tracks during the evening rush, prompting an evacuation of passengers. NTSB investigators recovered brake parts at two other locations on the train’s route.

“The potential for fatalities and serious injuries was significant,” NTSB Chair Jennifer Homendy said. “This could have resulted in a catastrophic event.”

Homendy urged other transit agencies that use cars from the same manufacturer, Kawasaki Rail Car, to check their fleets. The company also makes cars for the New York subway system and commuter rail lines in the New York region, although there was no indication Monday the problem had spread beyond Metro.

The shortage of trains created some of the most crowded rail cars and stations since the coronavirus pandemic was declared early last year. The investigation is a setback for Metro at a time when it was starting to see commuters return as more offices reopen.

The transit agency said it expected to continue operating with reduced service levels until at least Sunday.

“I want to assure our customers that their safety is driving every decision being made,” Metro General Manager Paul J. Wiedefeld said in a statement. “We apologize for the reduced service, and ask for our customers’ continued patience and support as we work to get Metro back to normal operations.”

Metro operated with about 40 trains Monday, offering what it called “basic service,” with trains departing about every 30 minutes. Metro has 748 of the 7000 series rail cars, which make up about 60 percent of its nearly 1,200-car fleet. Metro said Monday it would boost Red Line service to every 15 minutes, but other lines would stay on a 30-minute schedule. The Silver Line will operate between Wiehle-Reston East and Federal Center SW.

An investigation into a train derailment with no injuries grew Monday into an unanticipated crisis for Metro. The agency has been emphasizing the health of passengers and workers during the pandemic as it tries to rebound, but it now finds itself confronting a new challenge of attracting customers while battling a safety issue it has known about for years.

More than any other factor, Metro’s internal customer surveys show performance and reliability to be what riders desire most, and the loss of the 7000 series — which transit users widely favor over older cars — presents another blow.

U.S. Rep. Gerald E. Connolly (D-Va.) said Monday that information released by the NTSB showed Metro had failed for four years to act on a “potentially fatal” problem.

“This is unacceptable and could have resulted in a catastrophic tragedy,” he said in a statement. “I am demanding a plan of action from WMATA for how these issues will be addressed and rider confidence in system safety restored.”

The NTSB said Monday its investigation showed that wheels on the car that derailed had shifted outward on the axle, which caused it to leave the tracks. Metro had discovered multiple other cars with similar defects in recent years, Homendy said. It wasn’t clear how well known the problem was within the agency.

During a briefing Monday, Homendy said data indicated those problems were getting worse: Metro identified two failures in 2017, two more in 2018, four in 2019 and five in 2020. Before the derailment, the agency had found 18 failures this year, and inspections last week turned up 21 more. Homendy said about 200 still need to be inspected.

Those failures didn’t necessarily involve derailments, Homendy said, and were identified by measurements taken during routine inspections.

Joe Gordon, the NTSB’s chief investigator, said because wheels on the car involved in last week’s derailment had shifted outward, the train was bumped off the rails when it passed over a switch. Gordon said passing another switch knocked it back on, explaining how the train kept moving after the two previous derailments. But outside Rosslyn, where the train left the tracks for the final time, Gordon said, there was no second switch.

Homendy said Metro was aware of the problem and had been working with Kawasaki to resolve it. Metro said it has been in contact with Kawasaki about the problem since 2017.

“We have to still get information and data on how that was addressed, what discussions were taking place, what data they relied on for determining what the safety issues were and the decision-making in the resolution that they chose,” Homendy said.

Metro said when a faulty wheel assembly was discovered, the set of trains was taken out of service and the wheel setup was replaced.

Federal rules generally prohibit parties to NTSB investigations — which includes Metro — from releasing information about the investigation. Longtime Metro board member Tom Bulger, an alternate director on the board representing the District, declined to comment on the investigation or the NTSB’s findings, saying investigators instructed the board not to make public statements.

Michael Goldman, a Metro board member for eight years until his term ended in June, said he was never made aware of wheel or axle problems on the 7000 series train. Several current Metro board members didn’t respond Monday to a request for comment about the probe, while those who did deferred to Metro management or Board Chairman Paul Smedberg. Smedberg said in a statement that the board would take any actions necessary to ensure safety.

Russell G. Quimby, a retired NTSB investigator, said train car wheels are typically fixed to the axle using hydraulic presses employing tons of pressure and “shouldn’t ever move.” He urged the transit agency to be cautious about returning to service even those cars that don’t appear to be out of specification until investigators understand the root causes.

“When you’ve got that many wheel sets out of tolerance — something so critical and so basic — that should have set off alarm bells,” Quimby said. “Something’s really wrong.”

Metro’s 7000 series cars, built by Kawasaki at a plant in Nebraska, cost about $2 million each. They entered the system between 2015 and February 2020. Kawasaki did not respond to requests for comment Monday.

Vehicle manufacturers often join NTSB investigations, and NTSB spokeswoman Jennifer Gabris said representatives of the company had arrived in Washington.

Homendy said the NTSB would examine whether there are systemic problems related to Kawasaki rail cars, adding that the board was concerned about other possible problems with rail cars at other transit agencies.

“We may at some point issue an urgent recommendation, or I’d say if you’re a transit agency operating in the United States and you’re listening, make sure that you’re checking your cars, as well,” she said.

Several major transit agencies contacted by The Washington Post said they don’t use rail cars manufactured by the company.

The company’s website identifies it as a supplier of rail cars to the Metropolitan Transportation Authority’s subway and commuter rail systems in New York. In 2018, the company said it had reached a deal to supply as many as 1,612 rail cars to the MTA, a contract worth up to $3.7 billion. The cars are scheduled to start service next year.

The MTA said none of the cars Kawasaki is building for the agency have the same designs, specifications or wheel assemblies as Metro’s cars. The Long Island Rail Road’s bi-level passenger coaches built by Kawasaki have been in service since 1997 and have not had wheel failures, officials said.

“We will continue to work with Kawasaki to maximize safety and closely monitor the results of the NTSB and D.C. Metro investigations to ensure full compliance with federal car equipment standards,” MTA spokesperson Eugene Resnick said in a statement.

Kawasaki says it also supplies the Port Authority of New York and New Jersey’s PATH rail system. Scott Ladd, a Port Authority spokesman, said its trains are a different model and do not appear to be affected.

Metro apologized to riders Monday amid reports of waits reaching an hour on some Metrorail lines and images of crowded trains on social media.

“We understand the impact this decision has on transportation for the DMV area,” the agency tweeted. “We apologize for this reduction in service and the inconvenience this is causing our customers.”

Morning commuters raced to find open spots within the six-car trains — down from the standard eight cars.

Eric Perless, an economic policy analyst, was among a throng of people at the Gallery Place-Chinatown station waiting for a ride to work. When a train arrived, he said, the rear two cars filled quickly, forcing him to race up the platform to find an alternate spot.

He rushed into the train, only to have his backpack get caught in the door. As the Red Line train began to move, Perless pried the rail car’s door enough to pull his bag and laptop in.

“To say it was sardines in a pot would not be doing it enough justice,” he said. “By far the most packed I’ve ever seen the Metro.”

There didn’t appear to be a quick fix on the horizon to increase capacity.

A spokesman for the Washington Metrorail Safety Commission, an agency that Congress created to monitor Metro safety, said next steps for Metro are laid out in an order signed by the commission. The order said Metro will create a plan to detect and assess the problem, then come up with a process that can safely return the cars to service.

“We understand the operational impacts, but the focus is on safety,” commission spokesman Max Smith said. “We are awaiting Metrorail’s proposed plans as outlined in the order that will allow the safe return to revenue service of each 7000-series rail car.”

Smith said the commission was not notified of the wheel assembly problems before the NTSB investigation. Metro said it would review the results of its inspections with the commission and determine how to bring the series back into service.

The NTSB investigation found that the fourth axle of the car that derailed was out of compliance with 7000 series specifications, according to the order. Inspections of other cars not involved in the derailment found similar defects, the commission said.

The Blue Line train carrying 187 passengers derailed Tuesday before it reached the Arlington Cemetery station.

The transit system’s 6000 series, which makes up about 15 percent of Metro’s cars, has been sidelined since November, when the agency pulled the trains out of service after two train separations on the Red Line. Metro has only recently been returning the 184 cars to service.

Enrique Gutierrez, a spokesman for D.C. Public Schools, said the school system did not experience major setbacks from Metro’s decision to reduce rail service. He said the system would continue to excuse students who arrive late because of their Metro commute.